Others are going to disagree and recommend just going to a some carrier like Progressive directly.
However, what I recommend especially for new RVers is to use one of the brokers that specialize in RV insurance as they not only can answer questions you might have about insuring an RV but also have access to multiple carriers and will most likely have access to Progressive if it best fits your needs.
Here are the brokers/agents I brought up in another recent thread on the topic:
Some of the ones we've contacted or used in the past are:
Miller in Lake Oswego Oregon
Destination in Missoula Montana
<---really liked them
Explorer in Richfield Ohio
Farm & City (FCIS) in Forest City Iowa
They all have agents that specialize in RV insurance. Not saying that you can't do it on your own but using an agent that is familiar with what an RVer needs is much easier the first time around. There are other agents around the country who also specialize in RV insurance in addition to the ones listed above.
They are licensed in most states and can provide all types of insurance even if you still have property somewhere or need a liability umbrella (BTW, an umbrella policy is something that is really important for an RVer, IMO).
For us, liability insurance is the most important form of insurance to have. If something catastrophic should happen (and most likely, it won't), you don't want the risk of getting wiped out financially should an event occur that harms or causes death to another because of your negligence. Yes, we all think that we won't be so negligent to cause harm or death to another person but I for one, don't want to take the chance to be under insured if, God forbid, such an incident happens. Be sure to investigate the importance of having a liability umbrella policy to override your other policies.